Shifting Bass-Lines

Warming Favors Invasive Fish over Native Trout and Salmon

Lisa Hayward

A male smallmouth bass guarding his nest in Orgeon's John Day River. (Photo: Erika Sutherland)

CLIMATE change is rapidly altering freshwater systems across the Northwest as air temperatures warm, patterns of precipitation and snowmelt change, and droughts and wildfires increase in frequency and intensity. Many species, including the Chinook salmon, westslope cutthroat trout, and native bull trout are at risk.

Fortunately, managers still have a time to implement conservation measures with the potential to yield high future dividends. Here are three stories illustrating how climate researchers are helping.

Fighting for Chinook

Before Erika Sutherland was an aquatic ecologist, she was a fighter pilot.

“One of the rare opportunities I had was to look down on the world, all over the world, and see impacts, including how many political problems were caused by the sharing of water,” says Sutherland.

Sutherland’s aerial perspective helped her appreciate just how essential water is. Without it, not much survives, whether in human settlements or in ecological systems. Sutherland was inspired. She went on to study freshwater systems and what makes watersheds healthy.

Now a Northwest Climate Science Center (NW CSC) Graduate Fellow at the University of Washington, Sutherland has exchanged the pilot seat for a snorkel, which she uses to study rivers in the Pacific Northwest. Her current work focuses on the effects of climate-related stream warming on native salmon and helping managers strategize salmon conservation.

Climate projections indicate many salmon species will lose significant portions of their historic habitat as streams warm into ranges that the fish have not evolved to tolerate. At the same time, climate change provides an opportunity for introduced warm-water predators, such as the smallmouth bass, to expand their range into salmon rearing areas that were historically too cold for them.

In Oregon’s John Day River, where Sutherland conducts fieldwork, smallmouth bass have already expanded their range upstream, creeping into habitat critical for developing juvenile Chinook salmon. The juveniles don’t recognize the bass as threats, making them easy prey for the voracious predators.

Snorkeling for hours a day over large stretches of river—a dream job for Sutherland—the researcher is studying the biological constraints keeping smallmouth bass from expanding farther into colder waters—including temperature impacts on the hatch rate of eggs and growth of their fry. Sutherland says as she learns more about the mechanisms limiting the bass’s range expansion, she may be able to help curtail further invasions.

Understanding Climate Threats to Westslope Cutthroats

Farther west, in the remote upper Flathead River system of Montana and British Columbia, NW CSC and Great Northern Landscape Conservation Cooperative-funded U.S. Geological Survey biologist Clint Muhlfeld is documenting a similar story about pressures from climate change and an introduced species endangering a beloved native one.

Generations of fly-fishers have long prized the Flathead Basin for its cinematic backdrop of glacier-carved valleys and for its large, native westslope cutthroat trout, famous for their eagerness to take bait. Today, warmer air temperatures in the basin mean earlier spring run-off, lower summer flows, and warmer waters.

Native fish are losing habitat as the introduced rainbow trout expands its range. However, in this case, the threat from an introduced species is less about predation than something more amorous.

The rainbow trout are mating with the cutthroats and that’s not good for the species’ future survival, says Muhlfeld.

“Protecting genetic integrity and diversity of native species will be incredibly challenging when species are threatened with climate-induced invasive hybridization,” says Muhlfeld.

Once hybridization occurs, says Muhlfeld, there’s no way to return to genetically pure lines. According to his research, hybridization is having disastrous results for native westslope cutthroat trout. Progeny of interspecies liaisons are poorly suited to their environment, usually dying without reproducing.

Muhlfeld’s study, which is based on 30 years of research, has been the first to demonstrate the potential for climate change to decrease biodiversity by fostering cross-breeding between invasive and native species, a risk that was, until now, largely theoretical. Muhlfeld published his results in Nature Climate Change in December 2013.

“The evolutionary consequences of climate change are one of our greatest areas of uncertainty because empirical data addressing this issue are extraordinarily rare,” says Ryan Kovach, Muhlfeld’s co-author on the study. “This study is a tremendous step forward in our understanding of how climate change can influence evolutionary process and ultimately species biodiversity.”

Tracking Northwest Stream Temperatures

Chinook salmon and westslope cutthroats face many of the same challenges confronting a wide range of aquatic species across the Northwest. Successful management of our region’s freshwater resources requires a landscape-level approach and the coordination of many diverse groups.

Currently state, federal, tribal, and non-governmental organizations throughout the region are working together, producing powerful tools to help strategize conservation efforts. One example of this productive collaboration is the NorWeST Project’s stream temperature data archive.

Eight years ago, Daniel Isaak, a research fish biologist with the U.S. Forest Service, was attempting to analyze the impacts of wildfire on bull trout habitat when he realized that he needed a better temperature map.

“At the time,” explains Isaak, “there were no great analytical approaches available, although there were lots of data out there on summer stream temperature, along with data on where, why, and for how long these measurements were taken.”

So Isaak set about putting together a database of stream temperature records. The data he collected from multiple agencies turned out to be highly accurate and more abundant than he’d first expected.

Meanwhile the newly formed Great Northern Landscape Conservation Cooperative (GNLCC) was setting its first year priorities. Chief among them was investigating the impacts of climate change on cold-water fish species. However, as researchers at the GNLCC soon learned, one potential obstacle complicating the effort was a lack of stream temperature modeling.

Here’s the problem: stream temperatures don’t relate directly to air temperatures. Instead they depend on a suite of environmental factors, including snowmelt, flow rates, and the stream’s slope and aspect, to name a few. That means in order to better map future stream temperature projections you first need a model to relate air temperature projections to stream temperatures. For that the GNLCC turned to Isaak, funding him to expand his centralized stream temperature database and develop a computer model to better simulate stream temperatures.

Three years later, Isaak’s work—which later received funding from the North Pacific LCC—yielded an empirical statistical model capable of projecting stream temperature with high (90 percent) accuracy. The model combines downscaled, air temperature projections from global climate models with 11 publicly available predictor variables (such as elevation) in order to map future climate scenarios for all streams across the Northwest region.

Likely the largest of its kind, the stream temperature database Isaak and his team compiled is impressive, encompassing measurements from more than 234,000 kilometers (145,400 miles) of streams over the course of 20,000 separate field campaigns. More than 60 state, federal, tribal, and private resource agencies have contributed from across Oregon, Washington, Idaho, Montana, and Wyoming.

The end product is a series of “pretty maps”, as Isaak describes them, showing, among other things, how much habitat is left for native fish and how that habitat is likely to change over time. These maps highlight places that are big enough and likely to stay cold enough to support native fish in the future. Groups, including the Blue Mountain Adaptation Partnership and the Northern Rockies Adaptation Partnership (started by the University of Washington’s David Peterson and funded by the National Oceanic and Atmospheric Administration through the Pacific Northwest Climate Impacts Research Consortium), are using the maps to prioritize conservation and restoration efforts.

Looking to the Future

Ultimately the best hope for the Northwest’s native fish rests in the combined efforts of people such as Sutherland, Muhlfeld, Isaak, and the countless others across the region, monitoring, modeling, snorkeling, and planning. The good news is that the Northwest still holds places suitable as future refugia for salmon and trout, and people are acting now to mitigate climate impacts on native fish.

This colorful map shows average stream temperatures for central Idaho’s Salmon River Basin from 1993-2011 as predicted by the NorWeST temperature model. More than 234,000 kilometers (145,400 miles) of streams have been computer modeled across the Northwest. To aid natural resource managers with science-based decision-making, temperature data and other products from the models are publically available on the NorWeST website. The project’s database can be accessed here.